Turbidity comes from the Greek word turbid while nephelometric is the Greek word for cloud. In water treatment plants the required detection limit is approximately one part per million of particles with a detectable change of one part per billion of particles and is read in Nephelometric Turbidity Units, NTU. The specifications required for monitoring of water is set by each country, but all are similar in their quality control characteristics and performance. The cloudiness is caused by suspended solids in the fluid which may be organic material, clay, sand or other particulate matter. Bubbles are also seen as a particle and must be eliminated from the equation. In water treatment plants this measurement provides: (1) a measurement of filter effectiveness and (2) a surrogate method for determining the level of microbial contaminates embedded in the particulate matter in the incoming raw water and any microbial remnants in the filtered water. The greater the concentration of particles in the fluid, or turbidity, the higher the level of microbial contaminates embedded in the particles and the higher the required level of disinfection procedures. Treatment water plants walk a thin line between too little disinfection in protecting the community from harmful microbes and too much disinfection causing harm from disinfection by-products. Nephelometric turbidity is the EPA workhorse of all water treatment plants.
There are world-wide standard specifications for the measurement of turbidity in water treatment: (1) The USEPA 180.1 Nephelometric Method (2) 2130B Standard Methods for the Examination of Water and Waste Water and (3) the ISO 7027 International Standard Water Quality Determination of Turbidity 1999-12-15, (4) Hach 10133, (5) GLI Method II, (6) Mitchell Method M5331 LED, and (7) Mitchell Method M5271 laser. All specifications measure the intensity of scattered light from suspended particles in the fluid having a refractive index different from that of the sample fluid, measured at a 90 degree angle to the path of the incident light.
In order to satisfy the various wavelength requirements specified by different government agencies, a number of light sources have been EPA approved for nephelometric turbidity measurement and reporting:
USEPA 180.1-1Incandescent2200-3000K OriginalUSEPA 180.1-2LED860 nm GLI 4-beamUSEPA 180.1-3Laser660 nm photomultiplier tubeUSEPA 180.1-4LED525 nm Mitchell Method M5331USEPA 180.1-5Laser660 nm Mitchell Method M5271ISO 7027 basicLED860 nm InternationalISO 7027 alternateLED550 nm International
Each of these Methods has unique specifications. U.S. Pat. No. 7,142,299 states that each light source must have a sensor source receiver measuring voltages to detect differences in light sources, which are factory preset in order to determine if an LED or incandescent lamp is in use. It distinguishes type but not wavelength. In all current instrumentation, there is no design allowing for a plug-in method of exchanging only the light sources in a single sensor body or for automatically adjusting the system for matching algorithms to both source type and wavelength. The same patent also describes a sensor unit mounted on the cover inside the chamber. This arrangement is used by AquaSensor.
Many instruments either suspend the light source directly above the fluid with the scattered light detector submerged U.S. Pat. No. 6,307,630 or transmit and receive signals through a transparent cuvette which contains the fluid U.S. Pat. No. 5,446,544 or have transparent surfaces directly immersed in a fluid U.S. Pat. No. 7,142,299B2. Any difference in temperature between the fluid and interior of the sensor will result in fogging of the windows if any moisture is present. Efforts to control the moisture in the atmosphere behind the windows include use of desiccant dryers or fans U.S. Pat. No. 5,446,544. Desiccants are ineffective after a short period of time while other drying devices such as fans are unwieldy, only partially effective and expensive. There are no devices using a multiple, sealed, window construction for eliminating fogging in this context.
In order to measure low nephelometric levels of turbidity, the signal received from the scattered particles must be as free of extraneous signals as possible. The scattered light bouncing around inside the chamber induces background noise which cannot be distinguished from the turbidity response to actual particles. Several approaches are commonly used to limit the unwanted reflections ranging from a simple black surface, wavelength dependent coatings, or light trap apertures through which a beam passes into an isolated area where unwanted light is partially absorbed U.S. Pat. No. 6,307,630. These traps require a complex shape and sufficient space to operate U.S. Pat. No. 6,831,289 with inverted or non inverted cones U.S. Pat. No. 5,872,361 and U.S. Pat. No. 6,894,778 or use of plain cones U.S. Pat. No. 5,231,378. None of these approaches eliminate the small amount of returning light which is scattered throughout the chamber, nor does it eliminate the initial reflections which were only partially absorbed. There are no instruments utilizing cone diffuser with a tapered, cylinder shaped housing. There are no instruments utilizing a combination of multiple absorbing and reflective elements to attenuate the source light beam.
For on-line instrumentation measuring turbidity levels, bubbles which are seen as particulate matter must be removed from the equation. There are two standard approaches to the elimination of entrained air and bubbles: (1) separating fluid that contains entrained air from fluid which does not carry entrained air and (2) holding the sensor housing under pressure, preventing bubbles from forming. The bubbles are caused by temperature increase, pressure decrease or by previously entrained gases. Current designs for removal or prevention of entrained air and bubbles may actually generate additional bubbles from out-gassing due to pressure decrease or temperature increase. Pressurized systems are unable to eliminate existing entrained air and bubbles, but may prevent additional bubbles from forming.
Separation of gases may be done by passing the fluid over protrusions at operating or atmospheric pressure. U.S. Pat. Nos. 5,831,727 and 5,331,177 describe the general method. However, there is the problem of reduced pressure causing out-gassing and possibly allowing a temperature rise by using a fluid colder than the utility environment, which inadvertently can allow warming and out-gassing of bubbles. Use of a chamber which can be kept under pressure but allows bubbles to rise and vent out an upper port is shown in U.S. Pat. No. 4,740,709; however, this method can allow measurement to take place in an area of stagnation, thus providing false readings. U.S. Pat. No. 5,475,486 describes a bubble trap for use in a cuvette type operation, but requires separate drain ports at the top and bottom of the container. This method cannot be operated under pressure and requires a transparent cuvette chamber. U.S. Pat. No. 5,059,811 incorporates a bubble trap with a dam diverter and multiple fluid passages to direct the fluid flow. There are no current devices using a combination of fluid under pressure and fluid separator plate to set up different flow paths for fluid containing bubbles and fluid not containing bubbles.
The EPA has two standards for calibration: (1) primary calibration for EPA reporting and (2) secondary calibration for internal plant verification. EPA reporting requires a monitoring response to detect changes in the fluid's turbidity to approximately one part per billion of suspended particles. Accurate responses at this level are difficult to sustain using current instrumentation. The EPA has a number of concerns about the stability of current nephelometric turbidimeters: the incandescent light source tends to drift in intensity, the electronic components trend to drift, while the scattered light receiver deteriorates over time. The large majority of field units are of this type. There are currently no automated methods for maintaining instrument stability from drift or verifying operation; there are some designs with feedback loops which stabilize only the light source U.S. Pat. No. 5,828,458. The approach described in U.S. Pat. No. 6,842,243 attempts to detect changes in operation by periodically transmitting reflected light from the back of the source window optics to a reflected area on the back of the scattered light window optics but does not consider changes in reflected light going into the scattered light receiver caused by fouling, fogging, finger prints, etc.
Several methods of using manually inserted calibration apparatus for secondary calibration are in use but all suffer difficulties. U.S. Pat. No. 5,467,187 shows a pulsed system varying the position of using sealed standards in a cuvette requiring a pump and complex plumbing. Variation in position of the floating standards and complex processing in the change in signal limit the repeatability of this method.
Because of these problems, the EPA mandates frequent re-calibration. However, calibration is time consuming, requires a skilled operator, and stoppage of the on-line monitoring process; it is expensive, complex, and produces high operator error. In addition, current methods of calibration require exposing the sample fluid to contamination from the outside. A number of efforts have been made to approach the specifications as closely as possible. U.S. Pat. No. 5,467,187 attempts to use a water pulse with a sealed cuvette bouncing multiple standards in the detection area. Because of variability in placement position and surface contamination of scratches, fouling, and complex processing of signal, the units' variability response limits its application as a replacement for manual calibration. U.S. Pat. No. 6,307,630 describes a method of periodically adding a sample of uncontrolled source light to the test fluid via mirrors, optical switch and polarized filters in the sensor, but does not compensate for source lamp variation or film build-up on the windows, or negate the fluid's turbidity reading during calibration. There are a number of manual reference blocks acceptable to the EPA for secondary calibration, but none are automatic or protected in its environment free of scratches and finger prints, and all leave the sample water exposed to contamination, and most importantly can not be used for primary calibration for EPA reporting.
While there has been some advancement in turbidity devices since the 1950s, there remains a need to improve over the art, bringing the EPA specifications closer to available technology. The EPA is highly cautious about granting new Methods for EPA reporting and will only do so if there is sufficient improvement in safety, accuracy and advanced technology to warrant the EPA approval process. The process takes about four years to complete.